240 



GASTRULA-ARRESTED EMBRYOS 



( 1944 ) showed that a bit of nomial head endoderm ( rider ) suit- 

 ably mounted upon the surface of a yolk endoderm substratum 

 ( base ) would in due course embed itself in the latter; and he was 

 able to demonstrate that cells of the rider transformed into flask 

 cells as they infiltrated the cells of the base. Applying this method 

 to hybrids, Gregg and Klein (1955) found that presumptive 

 head endoderm riders from hybrid embryos embed in yolk en- 

 doderm bases with as much dispatch as those from normal em- 

 bryos. Their systems were not studied histologically, but as far as 



180»' 



90» 



0» 



■ 



/ 



PIPIENS 



HXBRIO 



20 



"to 60 80 

 HOURS DEVELOPMENT l8»C 



100 



Fig. 5. The angle of invagination in developing Rana pipiens and 

 hybrid embryos. (Reconstructed from data of Moore, 1946.) 



their evidence goes, it sustains the belief that hybrid dorsal lip 

 endoderm is capable of normal morphogenetic behavior. 



Archenteron Formation and Enlargement. Continued invagi- 

 nation of the pigmented streak just vegetal to the earliest dorsal 

 lip soon results in the formation of a cavity, the archenteron, 

 which in normal amphibian embryos enlarges continuously while 

 gastrulation proceeds (Fig. 4). Moore has devised a method for 

 obtaining a measure of the rate and extent of archenteron forma- 

 tion. It consists in obtaining the value of the angle subtended by 

 two lines in the median sagittal plane, both originating at a point 

 on the center of the blastocoel floor, and tangent respectively 



